The invention relates to a transfer system for transfer of fluids from a first floating or fixed structure to a second floating structure, the transfer system comprising a first and second duct section connected to the first and second sores respectively, and a substantially horizontal, submerged, third duct section interconnecting the first and second duct sections.
It is known to connect two floating offshore structures via a transfer duct system for conveying hydrocarbons from one structure to the other. One floating structure may be a production or storage structure such as a spar buoy, a semi-submersible structure, a fixed tower or a mooring buoy whereas the second structure may comprise a floating production storage and offloading vessel (FPSO), a shuttle tanker and the like. Such a system is described in Dutch patent application NL-A-8701849. In the known configuration, a production platform is anchored to the seabed via radial taut mooring lines, the platform being connected to a subsea well head via a riser. The production platform is connected to a mooring buoy via flexible duct sections. The duct sections are anchored to the seabed via tethers. The mooring buoy is connected to the seabed via a cable carrying at the end thereof a clump weight. The clump weight is anchored to the seabed via an anchor chain. The mooring buoy can freely drift within an area that is defined, by the length of the anchor chain between the clump weight and the sea bed. The taker that is moored to the buoy can weathervane around the buoy and is subject to drift in accordance with prevailing wind and current conditions.
From U.S. Pat. No. 4,339,002 a discharge manifold system is known wherein a flexible conduit extends vertically downwards from a production platform to below waterlevel, continues horizontally and extends vertically upward towards a mooring buoy which is anchored to the seabed.
The known systems have as a disadvantage that the duct sections may be subjected to bending/kinking or buckling due to currents which may displace the system sideways. In view of the connection of the shuttle tanker to the freely moving mooring buoy, the influence of the floating system dynamics on the transfer ducts, is limited but the system is relatively complex in view of the additional mooring buoy being required. Furthermore, in view of the freedom of movement of the tanker, there is a risk of the tanker damaging the transfer pipes.
An alterative option to connect two floating structures is to run the transfer pipes down to the seabed and back up in order to avoid curt and floating system-induced forces. Such a system however is not practical in deep water, for instance at depths of 1000 meters below sea level or more.
It is therefore ea object of the present invention to provide a transfer system in which the bending or buckling due to currents and floating system dynamics is reduced and which has a relatively small swing. It is another object of the present invention to provide a transfer system which can bridge a large distance between the interconnected structures. It is a further object of the present invention to provide a transfer system which can be produced in an economic manner.
Hereto the transfer system according to the present invention is characterized in that the horizontal member is near its ends provided with tensioning members oriented in a substantially vertical direction at least one tensioning member being inclined at an angle (xcex1) with respect to the vertical, a tensioning weight being connected at or near the ends of the horizontal member for providing a tensioning force on the third duct section.
Because of the inclination of at least one of the vertically positioned tensioning members, the ballast weight exerts a horizontal component on the substantially horizontal third duct section. Hereby it is kept from bending or buckling and has a reduced swing due to the restoring force created by the counterweight when it is offset from its equilibrium position. Furthermore, the system according to the present invention does not require additional mooring constructions and allows to use relatively long, substantially horizontal duct section, having a length of for instance 3000 meters.
With xe2x80x9csubstantially horizontalxe2x80x9d it is meant that the third duct section does not make a larger angle with the horizontal than at most 45xc2x0.
According to the invention it is possible to either integrate the tensioning member in either one of the first or second duct sections or embodying the tensioning member as a separate article.
In the first embodiment because of the tension, the related first or second duct section will generally extend according to a straight line. In the second embodiment the first and/or second duct section can have any shape.
This is dependent from its length relative to the length of the tensioning member as well as its weight. For example the related first or second duct section can comprise three parts, one substantial vertical part and other substantial horizontal part connected by a transitional part.
In one embodiment both first and second tensioning members are inclined with respect to the vertical, a tensioning weight being provided at or near each connecting point of the first and second duct sections with the third duct section. By using two tensioning weights, one at each end of the horizontal duct section, an even tension force can be applied on the horizontal duct section.
Preferably the first and second duct sections and/or tensioning members are attached to the third duct section via an articulation joint, such as for instance a flex joint or a pivoting joint. In one embodiment the duct sections are made of hard pipe which allows for a relatively economic manufacture. The use of hard pipe in this case is possible as the bending and buckling in the present system is reduced due to the tensioning effect of the weights. When hard pipe is used, the system of the present invention may be used in relatively large water depths such as 100-150 meters below sea level and deeper. It is possible to use however a combination of hard and flexible duct sections. Multiple transfer systems of the present invention may extend in a radial manner from a single floating structure, such as the spar buoy, to respective FPSO-tankers or buoys for export. The buoyancy of the tensioning weights may be adjustable for instance by ballasting the counter weights wit water or deballasting using compressed air. Additional weight could also be added or removed. The third duct section may be provided with buoyancy such as to have a neutral or even positive buoyancy in water.